We studied the effects of unilateral chronic anterior cruciate ligament (ACL) injury on bone size, bone mineral content (BMC), bone mineral density (BMD), soft tissue composition and muscle strength of the injured lower extremity in Japanese 21 men and 12 women aged 15 to 39 years. Bone area, BMD, BMC, lean mass and fat mass of lower extremity were measured using dual energy X-ray absorptiometry. The isometric and isokinetic muscle strength was assessed by an isokinetic machine.BMC, lean mass, circumference of the thigh and circumference of the lower leg of the injured lower extremity were significantly smaller than those of the intact lower extremity (p=0.0002, p<0.0001, p<0.0001, p=0.0131). In contrast, fat mass and %Fat of the injured lower extremity was significantly greater than that of the intact lower extremity (p=0.0301, p<0.0001). Bone area and BMD did not produce significant difference. These findings suggest that chronic insufficiency of ACL decreases BMC and lean mass of the injured lower extremity.